The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to real-time MRI, or MR fluoroscopy, in which data is acquired and images are reconstructed at relatively high update rates and low event-to-display latency.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement acquires an NMR signal, or "view" with a particular set of gradient values that sample a part of frequency space, or "k-space". The measurements are repeated with different gradient values until k-space is sufficiently sampled. The resulting set of received NMR signals are processed to reconstruct an image using one of many well known reconstruction techniques.
The ability to provide MR images to an operator in real time is becoming critical to more an more applications such as contrast bolus triggering, interventional MR and cardiac imaging. Real-time MR imaging requires continuous, efficient data acquisition as well as rapid, on-line reconstruction and display.
MR "fluoroscopy" was first demonstrated in U.S. Pat. No. 4,830,012 using a fast two-dimensional Fourier transform (2DFT) gradient-echo sequence with sequential phase-encode sampling. Using a view-sharing reconstruction technique, a given view may be used in the formation of more than one image. Views are acquired continuously, and image frames are reconstructed using recently acquired views and views acquired prior to the last reconstruction. This view-sharing allows more frequent image updating than if waiting for an entire set of new views to be collected. Similar view-sharing reconstruction techniques have subsequently been implemented for radial, spiral, and echo planar imaging (EPI) acquisitions.
Because low spatial frequency views (i.e. center of k-space) provide gross positional and contrast information while high-frequency views provide edge and detail information, temporal resolution is determined not only by the frequency of reconstruction, but also by the frequency at which the central region of k-space is sampled. Considering this, it was proposed that temporal resolution could be increased by sampling the central region of k-space more frequently than the remainder. Several investigators have implemented such differential-rate strategies for 2D imaging, either with or without real-time image reconstruction. Such methods reduce the time needed to acquire a frame of image data.
While high temporal resolution techniques have been pursued with great interest, the overall quality of true real-time imaging depends on more than just the frequency of data acquisition and reconstruction, defined as the "frame rate". More particularly, the display latency, or period of time between the occurrence of an event in the subject and the depiction of the event on a display should be as short as possible. This is not determined exclusively by the speed of acquisition and reconstruction hardware.